Pregnancy and early childhood are especially critical periods for child development. Both nature (genes) and nurture (environment), and particularly interactions between the two, are important influences. One way that the nature-nurture interaction can work is through environment affecting how a particular gene is expressed, either mitigating or reinforcing genetic differences between individuals. There are many studies that link health at birth to future outcomes. In this article, I review some of this research, identify several factors that help explain the trends observed over time, and link the implications of these findings for policy.